1 files changed, 67 insertions, 0 deletions
diff --git a/arch/x86/kernel/i8253.c b/arch/x86/kernel/i8253.c
new file mode 100644
index 000000000..2b7999a1a
--- /dev/null
+++ b/arch/x86/kernel/i8253.c
@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * 8253/PIT functions
+ *
+ */
+#include <linux/clockchips.h>
+#include <linux/init.h>
+#include <linux/timex.h>
+#include <linux/i8253.h>
+
+#include <asm/apic.h>
+#include <asm/hpet.h>
+#include <asm/time.h>
+#include <asm/smp.h>
+
+/*
+ * HPET replaces the PIT, when enabled. So we need to know, which of
+ * the two timers is used
+ */
+struct clock_event_device *global_clock_event;
+
+/*
+ * Modern chipsets can disable the PIT clock which makes it unusable. It
+ * would be possible to enable the clock but the registers are chipset
+ * specific and not discoverable. Avoid the whack a mole game.
+ *
+ * These platforms have discoverable TSC/CPU frequencies but this also
+ * requires to know the local APIC timer frequency as it normally is
+ * calibrated against the PIT interrupt.
+ */
+static bool __init use_pit(void)
+{
+	if (!IS_ENABLED(CONFIG_X86_TSC) || !boot_cpu_has(X86_FEATURE_TSC))
+		return true;
+
+	/* This also returns true when APIC is disabled */
+	return apic_needs_pit();
+}
+
+bool __init pit_timer_init(void)
+{
+	if (!use_pit())
+		return false;
+
+	clockevent_i8253_init(true);
+	global_clock_event = &i8253_clockevent;
+	return true;
+}
+
+#ifndef CONFIG_X86_64
+static int __init init_pit_clocksource(void)
+{
+	 /*
+	  * Several reasons not to register PIT as a clocksource:
+	  *
+	  * - On SMP PIT does not scale due to i8253_lock
+	  * - when HPET is enabled
+	  * - when local APIC timer is active (PIT is switched off)
+	  */
+	if (num_possible_cpus() > 1 || is_hpet_enabled() ||
+	    !clockevent_state_periodic(&i8253_clockevent))
+		return 0;
+
+	return clocksource_i8253_init();
+}
+arch_initcall(init_pit_clocksource);
+#endif /* !CONFIG_X86_64 */